Over one million people in the U.S. live with limb loss, with an estimated 100,000 new cases each year, over 80% involving the lower limb. Commercial prostheses are available with many different designs and features, at prices ranging from a few hundred dollars to over one hundred thousand dollars. Yet the prescription process is based mostly on subjective assessments and past performance, with no way to prospectively determine whether an increment in cost will yield a satisfactory improvement in quality of life for a given individual.
The most important mobility issues are discomfort, stability and fatigue. Persons with below-knee amputation choose a lower self-selected walking speed than able-bodied persons, and expend at least 20% more energy to walk at the same speed. Many advanced conventional prostheses have features such as higher elastic energy storage and return, and users generally prefer these feet for reasons of comfort. Nonetheless, the speed and energy cost discrepancies from non-amputees persist despite a wide range of complexity and cost in commercial prosthetic feet.
Recently, advanced foot prostheses have come to market promising to break this barrier by restoring one crucial component all passive prostheses lack: an ankle joint that can perform net positive work on the body. One such device (the BiOM T2 System) can improve walking mechanics, returning walking speed and energy expenditure to near-normal levels for some patients. It is unclear, however, whether all individuals will benefit, and possible benefits come at a steep cost: such devices are currently priced near $80,000, accessible to only the wealthiest or best-reimbursed patients. This would be a bad investment for any patient who does not realize major gains in quality of life, especially since these typically cannot be returned or resold following initial use. Emerging robotic prostheses like the BiOM intensify a longstanding dilemma in prosthetics practice: how can practitioners and insurance companies identify who will benefit sufficiently from increased performance to justify the higher cost of advanced devices? This problem has recently become more acute, as Medicare and other payers have identified cases of fraud, and in response have increased the demand for documentation to support classification of each individual's gait capacity. The argument is that current practice for assigning a K-level (KO to K4) relies too heavily on unreliable information such as prosthetist's opinion and the patient's stated activities and goals, and so can be manipulated, to the detriment of the payer.
Recent advances have added some nuance to the differentiation among K-levels, such as short in-clinic functional mobility tests or approximate activity classification based on time-binned step clustering. These tests include tasks such as freely-selected walking, standing and sitting transitions, climbing and descending stairs, navigating obstacles, and single-limb standing. However, all of these categorization methods have a common limitation: they are based on current mobility with the patient's current conventional prosthesis. They do not incorporate any information on how an individual patient will use and respond to a more advanced prosthesis, such as the BiOM T2. There is essentially no information available to help clinicians and payers determine whether a particular patient will benefit from an advanced prosthesis. There is therefore a high probability of suboptimal patient outcomes, economic inefficiency, and provider-carrier conflict during the prescription of advanced prostheses.
Robotic prostheses can improve locomotor performance for individuals who have restricted mobility due to lower-limb amputation. During walking, these devices can restore normal ankle and knee kinematics, reduce metabolic rate, and provide direct neural control of the limb. As robotic technologies improve, active prostheses are expected to enhance performance even further. Ankle inversion-eversion, or roll, is an important aspect of prosthesis function. Commercial prostheses typically include a passive inversion-eversion degree of freedom, either using an explicit joint or a flexure. This mitigates undesirable inversion moments created by uneven ground. Inversion moment has a strong effect on side-to-side motions of the body during human walking, and its pattern is altered among individuals with amputation. Side-to-side motions seem to be less stable in bipedal locomotion, particularly for amputees. Difficulty controlling inversion-eversion torque in the prosthetic ankle may partially explain reduced stability and increased fear of falling and fall rates among people with amputation.
Robotic prosthesis designs have begun to incorporate active control of ankle inversion-eversion. A tethered ankle prosthesis with inversion provided by a four-bar linkage and controlled by a linear actuator has been described, in which a plantarflexion degree of freedom is provided using a passive spring. A prototype device intended to provide both plantarflexion and inversion-eversion control using two motors and a gimbal joint has also been described.
The mass of prostheses with active inversion-eversion control is generally related to joint design. Linkages and gimbal joints often involve large parts with complex loading, resulting in increased strength and mass requirements. An alternative is suggested by the split-toe flexures in conventional passive prostheses and the actuation schemes in some powered ankle orthoses. During walking, peak inversion-eversion torques are of much lower magnitude than peak plantarflexion torques, and the majority of the inversion impulse occurs during periods of high plantarflexion torque. Coupling plantarflexion and inversion-eversion torque through the actions of two hinged toes might therefore provide sufficient inversion capacity, allowing an elegant, lightweight design.
Mechatronic performance in experimental prosthesis systems can also be improved by separating actuation hardware from worn elements. A tethered emulator approach decouples the problems of discovering desirable prosthesis functionality from the challenges of developing fully autonomous systems. Powerful off-board motors and controllers can be connected to lightweight instrumented end-effectors via flexible tethers, resulting in low worn mass and high-fidelity torque control. Such systems can be used to haptically render virtual prostheses to human users, facilitating the discovery of novel device behaviors that can then be embedded in separate autonomous designs. This approach can also be used for rapid comparison of commercial prostheses in a clinical setting. To be most effective, such prosthesis emulators should have high closed-loop torque bandwidth and lightweight, strong, accurately-instrumented end-effectors.
Torque control in robotic emulator systems can be improved with appropriate series elasticity. Adding a spring in series with a high-stiffness transmission can reduce sensitivity to unexpected actuator displacements imposed by the human. Unfortunately, this compliance also reduces force bandwidth when the output is fixed, because the motor must displace further when stretching the spring. In a tethered system, the flexible transmission itself is likely to have significant compliance, which might provide appropriate series elasticity.